Optical light pipe and microwave waveguide interconnects in multichip modules formed using adaptive lithography

ABSTRACT

HDI fabrication techniques are employed to form a variety of optical waveguide structures in polymer materials. Adaptive optical connections are formed, taking into account the actual position and orientation of devices which may deviate from the ideal. Structures include solid light-conducting structures, hollow light-conducting structures which are also suitable for conducting cooling fluid, and optical switching devices employing liquid crystal material. A &#34;shrink back&#34; method may be used to form a tunnel in polymer material which is then filled with an uncured polymer material that shrinks upon curing.

This is a division of application Ser. No. 08/037,833, filed Mar. 29,1993 (now U.S. Pat. No. 5,562,838).

BACKGROUND OF THE INVENTION

This invention relates generally to optical waveguides in polymermaterials and, more particularly, to optical waveguides formed employingfabrication techniques compatible with high density interconnect (HDI)fabrication techniques, including the use of adaptive lithography tocompensate for component misposition.

As disclosed in commonly assigned Eichelberger et al. U.S. Pat. No.4,783,695, and related patents such as those referenced hereinbelow, ahigh density interconnect structure offers many advantages in thecompact assembly of electronic systems. For example, a microcomputerwhich incorporates between thirty and fifty chips, or even more, can befully assembled and interconnected on a single substrate which is twoinches long by two inches wide by 50 mils thick. This structure isreferred to herein as an "HDI structure", and the variouspreviously-disclosed methods for fabricating HDI structures are referredto herein as "HDI fabrication techniques".

Very briefly, in systems employing this high density interconnectstructure, a ceramic substrate is provided, and individual cavities, orone large cavity having appropriate depths at the intended locations ofthe various chips, are prepared. Various components are placed in theirdesired locations within the appropriate cavity and adhered to thesubstrate by means of a thermoplastic adhesive layer.

A multi-layer high density interconnect (HDI) overcoat structure is thenbuilt up to electrically interconnect the components into an actualfunctioning system. To begin the HDI overcoat structure, a polyimidedielectric film, such as KAPTON® polyimide available from E. I. du Pontde Nemours & Company, about 0.0005 to 0.003 inch (12.5 to 75 microns)thick is pretreated to promote adhesion and coated on one side withULTEM® polyetherimide resin available from General Electric Company, oranother thermoplastic, and laminated across the tops of the chips, othercomponents and the substrate, with the ULTEM resin serving as athermoplastic adhesive to hold the Kapton film in place. (KAPTON is atrademark of E. I. dupont de Nemours & Co., and ULTEM is a trademark ofGeneral Electric Company.) Exemplary lamination techniques are disclosedin commonly assigned Eichelberger et al. U.S. Pat. No. 4,933,042.

The actual as-placed locations of the various components and contactpads thereon are typically determined by employing optical imagingtechniques. Via holes are adaptively laser drilled in the Kapton filmand Ultem adhesive layers in alignment with the contact pads on theelectronic components in their actual as-placed positions. Exemplarylaser drilling techniques are disclosed in commonly assignedEichelberger et al. U.S. Pat. Nos. 4,714,516 and 4,894,115, and Loughranet al. U.S. Pat. No. 4,764,485.

A metallization layer is deposited over the KAPTON film layer andextends into the via holes to make electrical contact to the contactpads disposed thereunder. This metallization layer may be patterned noform individual conductors during its deposition, or may be deposited asa continuous layer and then patterned using photoresist and etchingtechniques. The photoresist is preferably exposed using a laser which,under program control, is scanned relative to the substrate to providean accurately aligned conductor pattern upon completion of the process.Exemplary techniques for patterning the metallization layer aredisclosed in commonly assigned Wojnarowski et al. U.S. Pat. Nos.4,780,177 and 4,842,677, and Eichelberger et al. U.S. Pat. No. 4,835,704which concerns an "Adaptive Lithography System To Provide High Densityinterconnect". Any misposition of the individual electronic componentsand their contact pads is compensated for by an adaptive laserlithography system as disclosed in U.S. Pat. No. 4,835,704.

As an alternative to electrical interconnections, various forms ofwaveguide structures in optically transparent dielectrics such aspolyimides have been proposed. Examples are disclosed in the literatureand include R. Selvaraj, H. T. Lin and J. F. McDonald, "IntegratedOptical Waveguides in Polyimide for Wafer Scale Integration," Journal ofLightwave Technology, Vol. 6, No. 6, June 1988, pages 1034-1044; and B.L. Booth, "Low Loss ChannelWaveguides in Polymers," Journal of LightwaveTechnology, Vol. 7, No. 10, October 1989, pages 1445-1453. Relatedtechniques are disclosed in Booth et al. U.S. Pat. No. 4,883,743.

SUMMARY OF THE INVENTION

One object of the invention is to merge HDI technology and opticalwaveguide technology, particularly through the use of adaptivelithography techniques, to produce optical interconnects.

A related object of the invention is no adapt HDI fabrication processesto a variety of microchannel waveguide structures.

Another object of the invention is to facilitate connections to fiberoptic cables, including connections between fiber optic cables andelectro-optical devices, and connections between fiber optic cables.

Another object of the invention is to adapt HDI fabrication processes tothe manufacture of optical switches.

In accordance with an overall aspect of the invention, it is recognizedthat various well-developed HDI fabrication processes, includingadaptive lithography, are adaptable to the formation of waveguidestructures in polymers. Thus, a laser may be employed to mill awaypolymer materials such as KAPTON polyimide, ULTEM polyetherimide and thelike to form micro-channel light pipes and/or optical waveguides whichare integral to a multi-chip module. The laser milling process may beemployed either as a positive or a negative imaging technique. Thus thedesired channel profile may be formed below the polymer surface, orabove. In general, the laser is used to directly ablate a controlleddepth pattern of channels and canals, with laser power and profile beingcontrolled in order to vary the channel depth, as previously done in thecontext of HDI fabrication processes.

In accordance with a more particular aspect, the invention provides anadaptive method for making an optical connection to a device on asubstrate, where the device is non necessarily at a predetermined idealposition and orientation on the substrate. The method, for example, maycomprise making an optical connection between an external waveguide andthe device, between two devices, between a base waveguide at apredetermined location and a device, or making an optical connection toan electro-optical device which has an electrical connection comprisinga high density interconnect structure.

The method comprises the steps of determining the actual position andorientation of the device on the substrate, and then adaptively formingan optical waveguide optically coupled to the device. The opticalwaveguide is formed along a route adapted as required for a properoptical coupling to the device in its actual position and orientation.In one form, the step of adaptively forming an optical waveguideincludes the substeps of at least partially embedding the device in alayer of optical waveguide material, and then removing portions of thelayer of optical waveguide material, such as by laser ablation, to leavethe optical waveguide.

Alternatively, an electro-optical device may be integrated within a highdensity interconnect electronic module fabricated generally as describedin the above-identified patents. In accordance with this aspect, aportion of the multilayer interconnect structure is removed, such as bylaser ablation, to form a cavity opening therein. An electro-opticaldevice is then placed within the cavity opening and situated withinpredetermined tolerances from an optimal or ideal position andorientation. The actual position and orientation of the electro-opticaldevice is determined, and an optical waveguide optically coupled to theelectro-optical device is then adaptively formed. The optical waveguideis formed along a route adapted as required for a proper opticalcoupling to the electro-optical device in its actual position andorientation, by at least partially embedding the device in a layer ofoptical waveguide material, and removing portions of the layer of opticwaveguide material to leave the optical waveguide.

In accordance with another aspect of the invention, a fiber opticlightguide is fabricated by first providing a body of polymer material,for example a polyimide, and then forming a groove in the polymersurface. Preferably, the groove is formed by laser ablation, andadaptively positioned. Next, a liner is formed in the groove, the linerbeing of a polymer material having a relatively lower index ofrefraction. By way of example, the liner may comprise p-methylmethacrylate or polystyrene and be formed by chemical vapor depositionor atomic layer epitaxy. Finally, the groove and liner are filled with atransparent polymer material having a relatively higher index ofrefraction. The filling material, which comprises the actuallight-carrying waveguide structure, may comprise ULTEM polyetherimide orXU-218® polymer (available from Ciba-Geigy).

In one method for forming a structure in which light travels in a mediumof higher index of refraction, a solid body of polymer material isformed in a groove which is lined with a solid material having arelatively lower index of refraction.

In accordance with another embodiment, which may be termed a "shrinkback" embodiment, a polymer fiber optic waveguide is surrounded by agap, which may be an air or other gas-filled gap, or even a vacuum. Amethod for forming such fiber optic waveguide begins by providing asubstrate, such as a layer of polyimide, and forming a groove in thesubstrate surface, as by laser ablation. The groove is then filled withan uncured polymer material which shrinks upon curing, and which has anindex of refraction greater than that of air. The uncured polymermaterial may comprise, for example, polyester resin or epoxy resin. Thenthe polymer material within the groove is allowed no cure and shrink toform a solid light-conducting structure smaller than the groove, suchthat a gap is formed between the solid light-conducting structure in thegroove. Preferably, a cover layer is applied over the surface of thesubstrate to form a cover for the groove prior to allowing the polymermaterial within the groove to cure and shrink back, resulting in a gapbetween the solid light-conducting structure and the cover.

In accordance with yet another aspect of the invention, hollowlight-conducting tubes (with or without metal liners) are formed by, forexample, forming a groove in the surface of a body of polymer materialand then applying a layer of reflective material to surfaces within thegroove. A cover layer of polymer material including a reflectivematerial surface layer facing the groove is applied over the surface ofthe body to form a cover for the groove. After the cover layer isapplied, additional reflective material is preferably deposited onsurfaces within the covered groove, as by chemical vapor deposition forexample, to fill any voids in the layers of reflective material.

The groove in the surface may be formed in any of a variety of ways,such as: employing a laser to ablate the polymer material; depositing ametal mask layer on the surface, forming an opening in the metal masklayer to define the location of the groove, and selectively etching thegroove; or hot tool pressing.

Alternatively, a groove is formed in the surface of a body of Polymermaterial, such as by laser ablation, masking and selective etching, orhot tool pressing. A cover layer of polymer material is applied over thesurface of the body to form a cover for the groove, and finally a layerof reflective material is formed on surfaces within the covered groove.Chemical vapor deposition may be employed to form a layer of reflectivematerial on surfaces within the covered groove. The reflective materialis preferably a layer of metal, such as aluminum or gold.

A number of structures may be fabricated in accordance with theinvention, including a transmission structure comprising a hollowmicrotunnel in polymer material for conducting cooling fluid, and suchhollow microtunnels with a light-reflective liner comprising an opticaltransmission structure. In hybrid hollow structures, both a metalcurrent-conducting liner and a light conducting liner are included.

The techniques of the invention may also be employed to form an opticalswitch which includes a layer of polymer material having a channel withtwo ends formed therein, liquid crystal material within the channel, anda pair of electrodes for applying a voltage across the liquid crystalmaterial to control light transmission between the channel ends.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth withparticularity in the appended claims, the invention, both as toorganization and content, will be better understood and appreciated fromthe following detailed description taken in conjunction with thedrawings, in which:

FIG. 1 is a conceptual overview of optical interconnects in HDI modules;

FIGS. 2-8 depict steps in an adaptive method for forming a lightguiderod structure on a substrate, and the resultant structure;

FIG. 9 is a sectional side view depicting a laser diode adaptivelyconnected to an optical fiber;

FIG. 10 is a top view of the adaptively connected laser diode of FIG. 9;

FIG. 11 is a sectional side view of an alternative connection to a laserdiode;

FIGS. 12-14 are sectional side views depicting an adaptive method forintegrating an electro-optical device within an HDI structure, and theresultant structure;

FIG. 15 is a cross-sectional view of a optical waveguide structureformed within a liner of relatively lower index of refraction;

FIGS. 16A and 16B depict method steps in fabricating the structure ofFIG. 15;

FIG. 17A is a cross-sectional view of a shrink back solid light pipesurrounded by an air gap within a groove in a ceramic substrate;

FIG. 17B is cross-sectional view similar to that of FIG. 17A, butwherein the groove is in a polymer body;

FIG. 18 is a cross-sectional view of a hollow metallized tubeembodiment;

FIGS. 19A through 19D depict method steps for fabricating the structureof FIG. 18 wherein grooves are formed by laser ablation;

FIGS. 20A through 20E depict alternative steps for fabricating a hollowmetallized waveguide such as that of FIG. 18, wherein grooves are formedby masking;

FIG. 21 is a cross-sectional view of a hybrid structure comprising bothmetal and optical signal-carrying walls surrounding a hollow core;

FIG. 22A depicts a liquid crystal switch in longitudinal cross section;and

FIG. 22B depicts the liquid crystal switch of FIG. 22A in lateral crosssection.

DETAILED DESCRIPTION

Referring initially to FIG. 1, the use of optical interconnects withinan HDI-like module 30 is depicted in conceptual overview. Module 30includes a substrate 32 upon which are mounted three representative dies34, 36 and 38, which may comprise optical devices or electro-opticaldevices. Module 30 additionally includes a multi-die cavity 4C,including a plurality of individual devices situated therein and whichmay comprise an assortment of electronic and electro-optical devices(not individually shown). Although not illustrated, module 30additionally includes a high density interconnect (HDI) structure formaking electrical interconnections between the various components, andsupplying power thereto. HDI fabrication techniques are described in thebackground patents referred to hereinabove, and are described brieflybelow with reference to FIG. 12.

A particular feature of the FIG. 1 module 30 is the use of opticalwaveguides 42 interconnecting the various devices. It is a feature ofthe invention that the optical waveguides may be fabricated employingtechniques compatible with well-developed HDI fabrication techniques,and may be adaptively positioned by adaptive lithography, such asdisclosed in the above-identified Eichelberger et al. U.S. Pat. No.4,835,704, in view of the actual positions of the individual devices.

Thus, as variations on known and well-developed HDI fabricationprocesses, channels and other structures are formed in polymermaterials, using masking or direct laser ablation techniques. Forexample, a laser under program control may be employed to directlyablate a controlled depth pattern of channels and canals. Laser powerand profile are used to vary the channel depth, in a manner known in thecontext of HDI fabrication techniques, such as the exemplary laserdrilling techniques disclosed in the above-identified Eichelberger etal. U.S. Pat. Nos. 4,714,516 and 4,894,115, and Loughran et al. U.S.Pat. No. 4,764,485.

The resultant channel pattern resembles resist images normally used topattern metal in the HDI fabrication process. These channels are cleanedby any convenient method, such as plasma ashing, reactive ion etching,or the like, to form a smooth profiled surface conducive to lightpropagation. In addition, the channels can be formed at least in park byother techniques, such as hot tool pressing.

There are a variety of materials which may be employed in the practiceof the invention to fabricate optical waveguide structures, with indexof refraction being a significant parameter to consider in any selectionprocess. A number of such materials are identified in the followingtable:

                  TABLE                                                           ______________________________________                                        CANDIDATE MATERIALS FOR WAVEGUIDE APPLICATIONS                                Polymer            Index of Refraction                                        ______________________________________                                        TEFLON ® AF-1600 (DuPont)                                                                    1.29-1.31                                                  TEFLON PFA (DuPont)                                                                              1.34                                                       PMMA               1.49-1.56                                                  Epoxies            1.5-1.6                                                    Polycarbonate (GE) 1.573                                                      Parylene C (Novatran Corp.)                                                                      1.639                                                      Parylene N         1.661                                                      Parylene D         1.669                                                      Polyetherimide (GE)                                                                              1.641                                                      Polyimides:                                                                   KAPTON (Dupont)    1.66                                                       SPI-129 (MicroSi)  1.732                                                      PI-2555 (DuPont)   1.702                                                      SIXEF-44 (Hoescht) 1.627                                                      XU-218 (Ciba-Geigy)                                                                              1.614                                                      PI (Honeywell)     1.62-1.65                                                  PROBIMIDE ® 400 (Ciba-Geigy)                                                                 1.64                                                       ______________________________________                                    

As is known, the general rule for total internal reflection,particularly in the context of fiber optic cables having alight-transmissive core surrounded by a cladding material, is that thematerial in which the light travels should have a higher index ofrefraction than the cladding material. As the angle of incidenceincreases, a critical angle is reached, for example 45°, where totalinternal reflection occurs. Air or vacuum as a cladding is ideal, sincethe index of refraction for both is almost 1.0.

Nevertheless, light may be transmitted where the material in which lightis traveling has a lower index of refraction than the cladding material.In such situations, internal reflection becomes near total at angles ofincidence approaching 90°, that is, at near-grazing angles of incidence.

FIGS. 2-8 illustrate an adaptive process for making several types ofoptical connections, and the resultant structure. A negative imagingprocess is employed in the embodiment of FIGS. 2-8 to form a waveguideabove a surface.

FIGS. 2 and 3 depict a conceptual starting point wherein a pair ofelectro-optical devices 50a and 50b and another electro-optical device52 are mounted on a substrate 54, as is an end 56 of an externalwaveguide 58. The external waveguide may be a glass fiber opticwaveguide, or may comprise a polymer material. Visible onelectro-optical devices 50b and 52 are representative optical portsdesignated 60. Electro-optical devices 50a and 50b are, for purposes ofexample, to be optically coupled to each other, while electro-opticaldevice 52 is, for purposes of example, to be optically coupled towaveguide 58. Thus one of the device pairs 50a, 50b is an opticalsource, such as a laser diode, and the other is an optical detector,such as a photodiode. Device 52 may be either an optical source ordetector, depending upon the application.

Substrate 54 comprises two layers, a base layer 62, which may be made ofany suitable material, such as a ceramic, and a layer 64 of suitableoptical cladding material having a relatively lower index of refraction.Examples of optical cladding materials useful for layer 64 are p-methylmethacrylate (PMMA, index 1.49), polystyrene (index 1.58 to 1.66),tetrafluoroethylene (TFE, index 1.35), fluorinated ethylene propylene(FEP, index 1.338) and PFA TEFLON (index 1.338 to 1.340).

The electro-optical devices 50a, 50b and 52 are generally situated atrespective predetermined positions and orientations on substrate 54.However, because commercially-available pick-and-place equipment used toposition devices is subject to tolerance variations, devices 50a, 50band 52 are not necessarily exactly at their ideal positions andorientations on the substrate 54.

The actual positions and orientations of devices 50a, 50b and 52 onsubstrate 54 are determined by employing, for example, optical imagingtechniques, such as disclosed in Eichelberger et al. U.S. Pat. No.4,835,704. The position of waveguide 82 may also be determined in thismanner.

Next, and as depicted in FIGS. 4 and 5, devices 50a, 50b and 52, as wellas the end 56 of external waveguide 58, are at least partially embeddedin a layer of optical waveguide material 66 having a relatively higherindex of refraction. The layer of optical waveguide material 66 maycomprise, for example, ULTEM polyimide (index 1.66, XU-218 polyimide(index 1.614) or PROBOMIDE 400 polyimide (index 1.64). Layer 66 may beapplied by any suitable process, such as by applying an uncured resinand allowing the resin to cure to form a single layer, by spin coatingsuccessive 66, sublayers to build up the final thickness of layerthrough solvent evaporation techniques, by pouring hot liquefied opticalwaveguide material and allowing it to cool and solidify, or by filmlamination techniques.

Next, as conceptually illustrated in FIG. 6, a programmable scanninglaser, such as an argon ion laser with associated optics and scanningmechanism, generally designated 68, producing an energy beam 70, isemployed to form waveguide structures 72 and 74 above the surface ofsubstrate 54, as illustrated in FIGS. 7 and 8. In this example, anegative imaging technique is employed to ablate away material of layer66 where no waveguide is to remain. The resultant waveguide 72 couplesdevices 50a and 50b, while the resultant waveguide 74 couples device 52and external waveguide 58.

It is significant that the waveguide structures 72 and 74 in thisexample deviate from straight-line connections in order to accommodatethe actual positions of devices 50a, 50b and 52, and thus are eachformed along a route adapted as required for a proper optical connectionto devices 50a, 50b and 52 in their actual positions and orientations.

Although not illustrated, it will be appreciated that the resultantstructure of FIGS. 7 and 8 can be further processed by embedding theentire structure within a suitable waveguide cladding material having arelatively lower index of refraction. In absence of such cladding, airor other surrounding ambient media having a nominal index of retractionof 1.0 maintains the total internal reflection characteristics ofwaveguides 72 and 74.

Although a negative imaging technique is described above with referenceto the embodiment of FIGS. 2-8, it will be appreciated that a positiveimaging technique may equally well be employed, wherein a channelprofile is formed below a surface. Representative resultant structuresare described below with reference to FIGS. 15, 16A, 16B, 17A and 17B,but at this point it may be noted that the negative imaging process asjust described can be modified to implement positive imaging, beginningwith the step illustrated by FIGS. 4 and 5, by embedding devices 50a,50b and 52 in a layer of cladding material rather than a layer ofwaveguide material. The programmable scanning laser 58 of FIG. 6 is thenemployed to adaptively form channels in layer 66, and the channels arethen filled with a suitable light-conductive waveguide material. Duringsuch process, the optical ports 60 of devices 50a, 50b and 52 areprotected from the action of the programmable scanning laser 60 beam 70by masking with a photoresist, or other UV resistant material such as apolyimide, polyester, acrylate or epoxy, used as a resist.

FIGS. 9 and 10 depict a variation of the structure of FIGS. 7 and 8,wherein an optical semiconductor or electro-optical device, in thisexample a laser diode 80, is adaptively interconnected to an externaloptical fiber or optical waveguide 82. Examples of other electro-opticaldevices which may be similarly connected are multiplexers, decoders,senders and receivers. As has been discussed above, it is difficult,without adaptation, to properly align with a waveguide or optical fiber82 an electro-optical device such as laser diode 80, due to potentialmisposition of both the diode and the waveguide.

Laser diode 80 more particularly comprises a generally elliptical exitslit 84 which emits a beam on the order of one or two micrometers inwidth. Laser diode includes a cathode electrode 86 and an anodeelectrode 88.

Laser diode 80 is generally included in an HDI structure such asdescribed in the background patents enumerated above, which HDIstructure includes a KAPTON polyimide dielectric layer 90, and ametallized conductor 92 electrically contacting the diode anode pad 88through an adaptively-positioned via 94. Laser diode 80 is receivedwithin a cavity 96 formed in a ceramic substrate 98, and the bottom ofcavity 96 supports a conductive metal layer 100 extending up to thesurface of substrate 98 to facilitate electrical connection to cathodeelectrode 86. Similarly, a fiber well or channel 102 (FIG. 10) is formedin the substrate, generally aligned with exit aperture 84. Preferably,and if required to maintain total internal reflection, a layer 104 ofcladding material is provided within channel 102, generally comparableto cladding material layer 64 described in conjunction with FIGS. 2-8.

Depending on the relative sizes of device 89 and waveguide 82, as wellas the positioning of exit aperture 84, there may be a space 106 betweenwaveguide 82 and the subsequently-laminated dielectric layer 90. Space106 may be an air space, or may be filled with a waveguide claddingmaterial (not shown) having a suitable index of refraction. In order tomaintain air space 106, air or other gas under pressure may be injectedat an appropriate stage of the lamination process. Exemplary laminationtechniques are disclosed in Eichelberger et al. U.S. Pat. No. 4,933,042.

Connection between the external waveguide 82 and the exit slit 84 isprovided by an conical waveguide segment 108 adaptively formed, asdescribed above, along a route adapted as required for a proper opticalconnection between device 80 and waveguide 82.

In an alternative method of fabricating a structure similar to that ofFIGS. 9 and 10, optical fiber 82 is inserted into well or channel 102.Laser diode 80, preferably with optical waveguide segment 108 alreadyattached, is placed within cavity 96. Adaptive lithography may beemployed to align exit slit 84 and waveguide segment 108 prior toplacing laser diode 80 within the cavity. Waveguide segment 108 is thenheated by a laser or a hot gas reflow mechanism so that the waveguidesegment bonds with optical fiber 82, as well as with laser diode 80 ifnot already bonded.

FIG. 11 depicts another technique for interfacing a cladded opticalfiber 82 to laser diode 80. In FIG. 11, laser diode 80 is embodied inwhat is generally the previously-known HDI structure comprising aceramic substrate 120, a KAPTON polyimide layer 122 used as a spacinglayer, and KAPTON polyimide dielectric layer 90 supporting metallization92 and adhered to KAPTON polyimide spacing layer 122 by an ULTEMpolyetherimide adhesive layer 124. Optical fiber 82 is secured inposition within the structure by means of a suitable bonding adhesive126, and is supported near its end 128 by layers 130 of polyimide.

The actual connection between laser diode exit aperture 84 and opticalfiber 82 constitutes an adaptively formed waveguide segment 74,generally comprised of polymer material.

FIGS. 12-14 depict a variation on the adaptive method of forming alightguide rod structure shown in FIGS. 2-8, wherein an electro-opticaldevice is integrated within a high density interconnect module after thehigh density interconnect module is initially formed.

As a starting point, FIG. 12 depicts a high density interconnect module140 of the type generally described above in the background section, andincludes a ceramic substrate 142 having representative cavities 144formed therein by a suitable milling process, such as laser, ultrasonicor mechanical milling. Placed and adhered within cavities 144 arecomponents in the representative form of individual integrated circuitchips 146 and 148 having upper surfaces 150 with chip contact pads 152disposed thereon. In this example, chip 148 is an electro-opticaldevice, and includes an optical port 154 on its major surface.

HDI module 140 is completed by fabricating an interconnect structure inthe form of an overcoat 156. Briefly, a polyimide film 158, such asKAPTON polyimide about 12.5 to 75 microns thick, is pre-treated topromote adhesion, and is coated on one side with a polyetherimide resinor another thermoplastic (not shown). Film 158 is then laminated acrossthe upper surfaces 150 of chips 146 and 148, and across the upperunmilled surface of substrate 142, through its coated side. Thereafter,via holes 160 are laser drilled in KAPTON polyimide film 158 (and thecoated thermoplastic layer thereon) in alignment with contact pads 152on chips 146 and 148 to which it is desired to make contact. Buildup ofHDI overcoat 156 continues by forming a patterned metallization layer162 over KAPTON polyimide layer 158, extending into via holes 160 tomake electrical contact with chip contact pads 152. Additionaldielectric and metallization layers are provided as required (such aslayers 158' and 162') in order to provide all of the desired electricalconnections among chips 146 and 148. HDI overcoat 156 may include aprotective upper polymer layer 164.

As shown in FIG. 13, a portion of multilayer interconnect structure 156is then removed, to form a cavity opening 166 therein, the cavity bottomcomprising layer 158 of KAPTON polyimide. The cavity opening may beformed by any one of several processes, such as laser ablation, mask andRIE etch, mechanical milling or selective etching.

Next, as depicted in FIG. 14, an electro-optical device 168 is placedwithin cavity opening 166. Although electro-optical device 168 has anominal position and orientation, it is not necessarily in apredetermined ideal position and orientation, and the actual positionand orientation are accordingly determined by adoptive lithography asset forth in the aforementioned U.S. Pat. No. 4,783,695.

Subsequent processing proceeds generally as described above withreference to FIGS. 2-8, to adaptively form an optical waveguide 170connected to device 168. In this particular example, optical waveguide170 is connected to optical port 154 of device 148 through anappropriate optical via 172.

To provide electrical connection to device 168, a further dielectriclayer 158" is applied, so long as conditions for total internalreflection within the various waveguide structures are maintained. Layer158" has patterned metallization layer 162" contacting a contact pad 174through a via 176.

FIG. 15 depicts in greater detail a cross section of an opticalwaveguide structure formed by positive imaging techniques wherein awaveguide medium 170, such as a polyimide, is below the surface 172 of asubstrate 174, also of polyimide. In order to confine light by totalinternal reflection within waveguide 170, the waveguide is formed withina liner 176 of a material having a relatively lower index of refraction,such as p-methyl methacrylate or polystyrene. The upper surface 178 ofoptical waveguide 170 is exposed to air or other gaseous media having anominal index of refraction of 1.0, thus maintaining total internalreflection at the upper surface.

A process for making the structure of FIG. 15 includes the step,illustrated in FIG. 16A, of forming a groove or channel 180 in thesurface 172 of polymer substrate material 174. As described hereinabovewith reference to FIG. 6, an argon ion laser assembly 182 emitting acontrolled beam 184 is employed to adaptively form channel 180 in adesired position. In order to form a smooth profiled surface conduciveto light propagation, channel 180 is cleaned by any convenient methodsuch as plasma ashing, reactive ion etching, or the like.

Next, as depicted in FIG. 16B, liner 176 of the material having arelatively lower index of refraction is formed within groove 180 througha suitable process such as chemical vapor deposition or atomic layerepitaxy, indicated by particles 177. Thereafter, groove 180 and liner176 are filled with a transparent polymer material having a relativelyhigher index of refraction in order to form the resultant structure ofFIG. 15.

As an alternative to the low refractive index liner 176 of FIG. 15,FIGS. 17A and 17B illustrate alternative forms of a "shrink-back"embodiment wherein an optically transmissive light pipe is generallysurrounded by an air gap, or other gaseous gap, rather than by a solidmaterial. FIGS. 17A and 17B differ from each other primarily in thesubstrate employed; i.e., substrate 190 of FIG. 17A is ceramic, whereassubstrate 192 in FIG. 17B is a polymer material, such as KAPTONpolyimide. In both cases, a polymer cover comprising a KAPTON polyimidelayer 194 and an ULTEM polyetherimide adhesive layer 196 is formed overa channel 198 in substrate 190 or 192. Between substrate 190 or 192 anda polymer waveguide 200 is an air gap 202 which separates the waveguidematerial from the wall of channel 198. This yields an air gap 202 havingan index of refraction of very nearly b 1.0, with waveguide 200 beingformed of a shrink-away material of a selectable index of refraction.

Light-conductive waveguide 200 touches cavity walls 198 at variouspoints along its length, potentially resulting in some light loss atthose points. However, the total area of such contact points relative tothe overall surface area of waveguide 200 is relatively insignificant,and the resultant loss can be tolerated.

In methods of forming structures such as shown in FIGS. 17A or 17B,groove 198 is formed in substrate 190 or 192, using a processappropriate to the particular substrate material, such as laser ablationor milling. After cover 194 is applied, the resultant tunnel-likestructure is filled with an uncured polymer material which shrinks uponcuring and which has an index of refraction greater than that of air, toform waveguide 200. Examples of suitable polymer materials are epoxies,polyesters and acrylates. The polymer material within the groove ortunnel is accordingly allowed to cure and shrink to form the solidlight-conducting structure smaller than the groove, resulting in thegap. The degree of shrink back may be controlled through appropriateselection of the particular materials used.

Preferably a release agent (not shown) is employed to facilitate releaseof shrink-back waveguide material 200 from the walls of the groove ortunnel. Exemplary release agents, which are employed to coat the tunnelwalls prior to filling with uncured polymer material, are silicones andTEFLON polytetrafluoroethyiene. (Teflon is a trademark of E. I. dupontde Nemours & Company.)

The channels or tunnels can be filled after assembly of aneiectro-optical module. Tunnels can be pressure injected by employingmicro-piping, syringe injection, or the like. Vacuum is preferablyemployed to eliminate bubbles. The tunnels may be filled throughout amodule, from level to level, forming vertical vias, integral splittersand the like.

The embodiments described in detail up no this point have involved solidoptical waveguide structures. FIG. 18 illustrates in cross section ahollow waveguide-like structure 220 comprising a hollow microchannel ortunnel 222 lined with a reflective surface material 224, preferablymetal, such as gold, aluminum or silver. The lining or walls thus formedtypically comprise two sub-layers 226 and 228. As in the previousembodiments, the structure is formed within a polyimide substrate 230and a polyimide cover layer 232 is attached by means of a suitableadhesive layer 234. The funnel-like structure is thus surface-coatedwith metals such as gold, aluminum or silver, or appropriate dielectriccoatings of high reflectivity, to transmit light flux through tunnel 222by reflection without total internal reflection. The cross-sectionaldimensions of tunnel 222 are on the order of several mils by severalmils, with lengths ranging from several mils to several inches.

The hollow tube structure shown in FIG. 18 has two advantages inparticular. As one advantage, the tunnel structures may be employed tocarry fluid, either gaseous or liquid, for cooling electronic modules.(in the case of a liquid cooling fluid, optical transmission by means ofreflection would normally not be employed.) In fact, the tunnels of theinvention may be employed for cooling purposes alone, not necessarily incombination with the transmission of optical signals.

As a second advantage, because walls 224 may be comprised of metal,electrical signals may be carried by the same channels. Thus, forexample, power and ground currents may be carried through metal walls224, and the metal walls may as well comprise signal conductors.

Related to this, the waveguide structures thus formed may be employedfor transmitting high frequency electrical energy or signals, such asmicrowaves.

FIGS. 19A through 19D depict one process for fabricating a hollowwaveguide structure such as that shown in FIG. 18.

In FIG. 19A, an argon ion laser (not shown) operating at 351 nmwavelength is used to ablate a channel 242 in the surface 240 of a bodyof polymer material 230, forming a micro-channel typically a few milswide and a few mils deep. Channel 242 is further cleaned after ablationusing appropriate plasma ashing or reactive ion etching processes.

Next, with reference to FIG. 19B, layer 226 of reflective material isapplied to surfaces of the groove. Layer 226 may comprise titanium,gold, or other appropriate metal, including sublayers (not shown) forthe desired light transfer. Metal layer 226 is deposited by any suitableprocess such as vapor deposition or sputtering, and is patternedselectively in channel 242 either by photolithographic techniques, or bymechanically lapping the entire top surface leaving metal only in thechannels.

As an initially separate element, a layer of KAPTON polyimide 244 isalso metallized, for example with Ti/Au, and then laminated over thesurface 240 of polymer body 230, as depicted in FIG. 19C. The laminationis carried out with the metallized strip 246 face down, i.e., contactingpolymer 230, through a layer 248 of ULTEM polyetherimide adhesive.

This process as thus far described may result in voids 249 on thesurface metallization within tunnel or microchannel 222. Accordingly, toproduce the structure of FIG. 19D with substantially continuousmetallization, an additional layer 228 of gold, for example, is formed.This may be accomplished by purging an organometallic gold compound suchas cold hexafluoroacetylacetonate, which has a vapor pressure ofapproximately 0.7 torr at room temperature, through tunnels 222, andhealing the solid compound adore its decomposition point. Gold is thusdeposited on the inner surfaces of the channels, and other volatilebyproducts are purged out of the tunnels. Subsequent purging by air orother gas, or a liquid, may be done to clean the channels after thepurging with the organometallic compound. This approach provides a wayof metallizing the entire inner surfaces of the tunnels ormicrochannels. Laser or other heating from above can be used to locallyheat-assist decomposition of the gold compound, and pressure may beintroduced to prevent the tunnels from collapsing.

Rather than pre-metallizing KAPTON polyimide cover layer 244 withmetallized strip 246 as shown in FIG. 19C, chemical vapor deposition maybe employed as the only means for depositing gold on the interiorsurfaces of the tunnel. Such as process is represented in FIGS. 20Athrough 20E, which also depict an alternative method for forming thegroove.

Thus, as shown in FIG. 20A, a metal mask 252 is deposited on the surfaceof a polymer substrate 250 which may comprise KAPTON polyimide. Anopening 254 is then formed in metal mask 252, as shown in FIG. 20B,which opening 254 defines the location of the subsequently-formedgroove. Metal mask 252 may comprise sputtered titanium. A photoresist(not shown) is coated on the titanium layer, and patterned using anargon ion laser to form opening 254 in the mask.

The structure is then placed in a reactive ion etching chamber, andchannels are etched in unprotected regions of substrate 250, producing achannel 256 such as shown in FIG. 20C. Mask layer 254 is also removed.

As illustrated in FIG. 20D, a second layer 260 of KAPTON polyimide islaminated over channel 258, again employing an ULTEM polyetherimideadhesive layer 262. Thereafter, as shown in FIG. 20E, chemical vapordeposition is employed to form a gold lining 264 inside tunnel 258.

As alternatives to gold, various organic and inorganic flow-throughcoatings can be used to produce the desired reflective optional tube.Silver nitrate solutions can be coated onto a polymer surface, or microdoped into the polymer directly, and then later laser-heated, hot gassedinside the tubes, or E-beam bombarded to form mirror reflectingsurfaces.

FIG. 21 illustrates a hybrid structure 270 which may be fabricated inaccordance with the techniques of the invention. Structure 270 may begenerally characterized as a metallized tunnel with an optical liner forlight transmission and center cooling. Thus, structure 270 includes alaser-channeled polymer substrate 272 with a top cover 274 of Kaptonpolyimide laminated by means of a layer of Ultem adhesive 276.

A hollow tunnel 278 includes two metallized lining layers 280 and 282,generally as described above with reference to FIGS. 19A through 19D,for example. Sublayer 280 may comprise titanium and layer 282 maycomprise copper, for example.

Formed over layer 282 is an optical signal-conducting layer 284,including cladding, comprising a polymer material having a relativelyhigh index of refraction, formed by a process such as chemical vapordeposition (CVD) or atomic layer epitaxy (ALE).

Finally, particularly in cases where center liquid cooling is employed,there is an interior layer 286 of waveguide cladding material, alsoformed by a process such as ALE, CVD, or spin-on or spray-on coating.

In structure 270 of FIG. 21, electrical signal currents or power supplyconnections may be carried through metal layers 280 and 282, whileoptical signals are carried through the sleeve-like optical waveguidematerial 284. The hollow center 278 of waveguide structure 270 is thenavailable for cooling.

The order of the layers can be reversed, or further layers can beemployed. For example, layer 284 may comprise waveguide claddingmaterial and layer 286 may comprise waveguide optical transmissivematerial, particularly where a gaseous fluid (having an index ofrefraction of approximately 1.0) is employed for center cooling.

FIGS. 22A and 22B, represent in longitudinal and lateral cross section,respectively, a microtube liquid crystal switch 300. In general, liquidcrystal switch 300 resembles an optical waveguide of the type employinga solid light transmission medium but, instead of a material which isalways transmissive to light, a liquid crystal solution is employedwhich, through application of external electric fields, may in effectbecome alternatively transmissive, non-transmissive, or partiallytransmissive, for example through changes in index of refraction.

To form liquid crystal switch 300, a layer 302 of KAPTON polyimide islaminated to an HDI interconnect structure, represented only as asubstrate 304, using a low temperature thermoplastic adhesive (notshown). The lamination is carried out at 150° C. using a polyesteradhesive (such as T-320 available from Sheldahl Corporation, Northfield,Minn.). Large portions of this layer are ablated away, so as to createwells, using an argon ion laser. Electro-optical devices, represented inFIG. 22A as optical source 306 and optical detector 308, are then bondedin these wells, and electrically connected to an HDI circuit.

A microchannel 310 is then formed as described hereinabove. However,rather than employing a solid light transmissive material, a liquidcrystal material 312 between two electrodes 314 and 316 is employed. Thetotal length of the liquid crystal switch is kept relatively small, onthe order of a few millimeters, and fast switching times are provided byutilizing a polymer-dispersed liquid crystal electro-optic effect. Whena voltage is applied through electrodes 314 and 316 across layer 312,the orientation of the liquid crystal molecules is changed, resulting ina change in refractive index which blocks light from entering theoptical detector. Relatively fast switching times are possible becausethe effective liquid crystal thickness is extremely small. Responsetimes of significantly less than a millisecond are possible.

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that numerous modifications and changeswill occur to those skilled in the art. It is therefore to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within a true spirit and scope of the invention.

What is claimed is:
 1. An adaptive method for making an optical couplingto an electro-optical device on a substrate and electrically connectedin a high density interconnect structure including at least one layer ofpolymer dielectric material bonded to a major surface of theelectro-optical device and a metallization layer over the layer ofpolymer dielectric electrically connected through a via in the polymerdielectric layer to a contact pad on the electro-optical device, saiddevice being situated within predetermined tolerances from an idealposition and orientation on the substrate, the method comprising thesteps of:determining the actual position and orientation of the deviceon the substrate; and adaptively forming an optical waveguide opticallycoupled to the device, the optical waveguide being formed along a routeadapted as required for a proper optical coupling to the device in itsactual position and orientation.
 2. The adaptive method in accordancewith claim 1, wherein the step of adaptively forming an opticalwaveguide comprises the further steps of:at least partially embeddingthe device in a layer of optical waveguide material; and removingportions of the layer of optical waveguide material to leave the opticalwaveguide.
 3. The adaptive method in accordance with claim 2, whereinthe step of removing portions comprises laser-ablating the portions. 4.The method in accordance with claim 1, wherein the step of adaptivelyforming an optical waveguide also comprises the step of forming thewaveguide with proper optical coupling to an external waveguide in itsactual position and orientation.
 5. The adaptive method in accordancewith claim 1, wherein the step of adaptively forming an opticalwaveguide further comprises forming the waveguide with proper opticalcoupling to a second device in its actual position and orientation, thesecond device being situated on the substrate.
 6. The adaptive method inaccordance with claim 1, wherein the step of adaptively forming anoptical waveguide further comprises forming the waveguide with properoptical coupling to a base waveguide, said base waveguide being situatedon substrate at a predetermined location and with a predeterminedorientation.
 7. An adaptive method for integrating an electro-opticaldevice within a high density interconnect electronic module, said methodcomprising the steps of:providing a high density interconnect moduleincluding a substrate containing at least one cavity, said modulefurther including a plurality of integrated circuit chips disposed inthe cavity such that major surfaces of the chips are substantiallycoplanar with portions of the substrate surrounding the cavity, and amultilayer interconnect structure including interleaved layers ofdielectric material and conductive material disposed over the integratedcircuit chips and the substrate for establishing electricalinterconnections; removing a portion of the multilayer interconnectstructure to form a cavity opening therein; placing an electro-opticaldevice within the cavity opening and positioning said device withinpredetermined tolerances from an ideal position and orientation;determining the actual position and orientation of the electro-opticaldevice; and adaptively forming an optical waveguide optically coupled tothe electro-optical device, said optical waveguide being formed along aroute adapted as required for proper optical coupling to theelectro-optical device in its actual position and orientation, saidoptical waveguide being formed by at least partially embedding thedevice in a layer of optical waveguide material, and removing portionsof the layer of optical waveguide material to leave the opticalwaveguide.
 8. An adaptive method in accordance with claim 7, wherein thestep of removing portions comprises laser-ablating said portions.